Satellite communication access systems in which multiple terminal devices transmit to a base station via a nongeostationary satellite such as a quasi-zenith satellite in code division multiple access (CDMA) are known. On the other hand, techniques using orthogonal codes as a spread code sequence for CDMA communication are known.
For example, Non Patent Literature 1 describes an orthogonal Gold code sequence as a spread code sequence for code division multiple access (CDMA) communication. The Non Patent Literature 1 states that the generated orthogonal Gold code sequences are orthogonal to each other with a shift of 0.
Patent Literature 1 discloses a satellite communication system conducting data communication between a terminal device and a base station via an artificial satellite. The satellite communication system of the Patent Literature 1 generates transmission synchronization signals dividing reference synchronization signals obtained using the global positioning system (GPS) and synchronized with the current time into multiple time divisions, and spread code generation signals dividing the transmission synchronization signals into multiple time divisions. Upon transmission of data, a random number is generated and the data transmission starts when the number of transmission synchronization signals following reference synchronization signals becomes equal to the value of the random number. Furthermore, using spread codes sequentially output in sync with spread code generation signals, the modulated wave based on the transmission data is spread in spectrum to generate transmission signals.
In the random access control method disclosed in Patent Literature 2, the base station notifies the terminal devices of given delay time information. The terminal devices spread their transmission data with the same spread code, and set and transmit the spread transmission data of which the delay time is individually adjusted based on the notified delay time information in a specified slot.
Furthermore, Patent Literature 3 discloses a technique used in bidirectional satellite communication between a parent station and a mobile unit via a nongeostationary satellite such as a quasi-zenith satellite for frequency-compensating the carrier wave Doppler frequency shift caused by movement of the nongeostationary satellite based on orbit information of the nongeostationary satellite and location information of the mobile unit.
Non Patent Literature 2 discloses a basic study on bidirectional communication systems using a nongeostationary satellite. The Non Patent Literature 2 refers to prospects of using a quasi-zenith satellite or GPS satellite and transmitting very short messages such as someone's safety information via the satellite (quasi-zenith satellite). The Non Patent Literature 2 also discloses compensation of the carrier wave frequency deviation using GPS signals and compensation of the transmission delay difference using GPS signals.